This invention relates generally to measuring instruments, and more particularly to an improved head assembly for a gage for comparing the inside diameter of a bore.
A bore gage is an instrument designed to compare the inside diameter of a cylindrical bore formed in a workpiece with a master ring. A machinist who drills or cuts a bore in a workpiece will normally perform frequent precise measurements of the diameter of the bore to insure that the finished diameter corresponds precisely with the specified diameter. While calipers may be used for this purpose, bore gages are more convenient, quicker and oftentimes more precise.
A typical bore gage includes a head having a pair of angularly disposed centralizing contacts, one contact on either side of a center line disposed perpendicular to the longitudinal axis of the instrument. At one end of the center line is an extensible stem, and at the opposite end of the center line is a sensing contact. The matched pair of centralizing contacts, and the stem locate the bore gage head assembly along the center line of the bore to be checked so that the center line of the head assembly is positioned exactly along the center line of the bore, rather than a chord. In that position, the sensing contact, which is permitted to move within a limited range of motion, makes a measurement precisely at the diameter of the bore. The sensing contact is operatively connected to a mechanism which drives a dial indicator, LVDT amplifier or the like. In one type of bore gage, the sensing contact transmits its motion to a right angle transfer mechanism which in turn transmits that motion via a 1:1 ratio to a dial indicator or LVDT amplifier. Such a bore gage is usually set to a ring standard of known size. The deviation from that standard is then displayed on the dial indicator, an LVDT amplifier or the like.
In one type of bore gage, the centralizing contacts and the sensing contact are disposed on the ends of associated plungers that are disposed on the head assembly with a very precise slip fit. Both the centralizing contact plungers and the sensing contact plungers move in a direction parallel to their direction of elongation or parallel to their central axes. Typically, the clearance between the outside diameter of each plunger and the mating inside diameter of the plunger bore in the head assembly is 0.0003 inches to 0.0007 inches. Because of this very small clearance, any coolant, grit or chips that are left in the cylindrical bore being measured potentially work their way into the plunger mechanisms and inhibit the smooth motion of the plungers. Because the smooth motion of all of these plungers is absolutely essential for accurate bore diameter measurement, the presence of any such contaminants could have a severe impact on measurement accuracy. This problem has been compounded by a general industry trend toward water-based coolants as opposed to oil-based coolants used in the machining of the bores to be measured. Water-based coolants tend to have a corrosive effect on the steel plungers of the head assembly. Also, water-based coolants tend to leave a gummy residue on the plungers, which adversely affects the otherwise smooth motion thereof. While the bore a gage head assemblies can be disassembled and cleaned, this cleaning process is relatively time-consuming, and it results in downtime for the bore gages and lost worker time.
Examples of prior art mechanisms in which centralizing contacts and the sensing contact are mounted on plungers are described in U.S. Pat. Nos. 2,095,405; 2,547,364; 2,587,099; 2,601,496; 2,443,881; 2,443,880; and 2,429,511. Other patents that relate to bore gages that do not have plunger mechanisms include U.S. Pat. Nos. 4,419,829; 4,419,830; and 4,045,877.
The foregoing drawbacks of prior art bore gages using plungers are overcome by the present invention, which includes, in one aspect, two centralizing contacts disposed on an externally mounted pivot mechanism which is attached to the bore gage head by a single pivot.
In another aspect of the invention, the sensing contact and the mechanism for transferring the motion of the sensing contact to the plunger for operating the dial indicator or LVDT amplifier are combined into a single lever-type unit pivotally mounted on the bore gage head assembly. The sensing contact is disposed between the two centralizing contacts and is pivotable about a different axis.
In another aspect of the invention, the centralizing contacts, and the sensing contact have mechanisms which are exposed and may be readily cleaned without disassembly.
In a preferred embodiment, cutouts are provided on the sides of the bore gage head assembly to allow access to the mechanism for the sensing contact by a solvent or the like to permit rinsing and cleaning thereof without disassembly. The cutouts also permit an air stream from a pressurized air hose to be applied directly to the point at which the sensing contact mechanism engages the bottom of the plunger for operating the indicator. In another aspect, springs that bias the centralizing mechanism are external to the bore gage head assembly and therefore are easily replaced, cleaned and serviced.
In yet another further aspect of the invention, a set screw is provided in the head assembly which allows adjustment of the sensing contact measuring range. The set screw restricts the travel of the lever unit to the desired range.
In yet another further aspect of the invention, an external stop is disposed on the bore gage head assembly which cooperates with the pivot mechanism to limit movement of the centralizing contacts.
In yet another further aspect of the invention, the bottom bushing for the extension plunger for operating the indicator unit is spaced from the contact point with the sensing contact mechanism a sufficient distance to prevent coolant and debris from adversely affecting movement of the extension plunger.
In yet another further aspect of the invention, the sensing contact and the centralizing contacts comprise tungsten carbide balls.
In an alternative embodiment of the present invention, a hand operated mechanism is provided for retracting the centralizing contacts during entry of the head assembly into a bore to be measured.
As a consequence of the foregoing features, the bore gage of the present invention may be readily cleaned and repaired without extensive disassembly to permit smooth motion of all the elements essential for an accurate bore measurement.